The present invention relates generally to a brake control system for an automotive vehicle for controlling application and release of brake pressure in order to prevent the vehicle from skidding upon a rapid application of the vehicle brakes, as in an emergency situation. More specifically, the invention relates to an anti-skid brake control system for controlling the deceleration rate of the vehicle wheel r.p.m. relative to vehicle speed corresponding to friction between the wheel tread and road surface, in order to prevent the vehicle wheel from locking and thereby to prevent the vehicle from skidding.
Upon braking of a moving vehicle and the like such as an automotive vehicle, a vehicle wheel is apt to be locked to cause skidding. This will necessarily create an unstable condition in the controlled motion of the vehicle. Wheel lock-up may cause such a loss in directional stability as to result in an uncontrolled skidding, while, at the same time, the presence of locked wheels generally increases the distance required to stop the vehicle due to a reduced coefficient of friction which occurs while skidding under most road conditions. If skidding can be prevented, the vehicle can usually be stopped with greater safety in a shorter distance. Therefore, various brake control systems have been developed for preventing the wheels from locking, thereby preventing the vehicle from skidding. General and typical construction of such a brake control system has been described in U.S. Pat. No. 3,897,114, entitled "SKID CONTROL SYSTEM" to Ronald S. Scharlark. The U.S. Patent discloses a brake control system for controlling the braking of a wheeled vehicle to prevent skidding. The system functions to relieve the braking force applied to the vehicle wheel. The system is effectively responsive to a critical slip signal. The signal is generated in response to a sensed difference between a hypothetical vehicle deceleration, as approximated by a decreasing ramp signal, and the vehicle wheel r.p.m. The comparison is made on a differential basis to provide an output signal which is utilized in controlling an output gate. The braking force is reapplied upon the sensing of a positive wheel acceleration signal and a change in the sign of the rate of change of wheel acceleration from a positive to a negative value. During this period, the skid signal is ineffective to control the brake force.
As is known by those skilled in the art, when rapid braking is applied to a vehicle, a maximum braking effect can be obtained by providing approximately a 15% slip rate for the vehicle wheel with respect to the road surface, since the friction between the wheel tread and road surface is maximized at that rate. Accordingly, upon emergency and rapid brake operation, it is preferable to control wheel r.p.m. relative to the vehicle speed so that it becomes about 15% lower than the vehicle speed. Namely, the brake control system operates to control the deceleration rate of the wheel r.p.m. with respect to the vehicle speed so that the wheel r.p.m. is not excessively decelerated relative to the vehicle speed. Such operation is provided to avoid locking of the wheels and resultant slipping on the road surface. In practice, when the wheel r.p.m. is decelerated to be about 15% lower than the vehicle speed, a target wheel r.p.m. is determined based on the wheel r.p.m. and on a predetermined friction coefficient. Corresponding to the determined target wheel r.p.m., the deceleration rate of the wheel r.p.m. is controlled to change the actual wheel r.p.m. to approach the target wheel r.p.m. Thus, since the deceleration rate of the vehicle depends on friction between the wheel tread and the road surface, the target wheel r.p.m. is determined based on the vehicle speed and the friction coefficient.
In actual operation, the braking fluid pressure applied to the brake device of each wheel, i.e., to each wheel cylinder, is relieved in response to decelerating of the wheel r.p.m. to a lower speed than the target wheel r.p.m. Upon the occurrence of such a condition, the braking fluid pressure is again applied to the brake device of each wheel. By repeating this operation, the vehicle can be gradually and stably decelerated without causing locking of the wheel and therefore without causing wheel skidding on the road surface.
In the conventional system, the friction coefficient between the wheel tread and the road surface is presumed to be a constant value which is determined based on general road surface conditions. However, the actual friction coefficient of the wheel tread and the road surface varies considerably depending on wheel tread wear and the road surface conditions. If the actual friction coefficient is different from that of the presumed and predetermined value, the target wheel r.p.m. determined based on the predetermined friction coefficient may not correspond to the actual vehicle speed.
For situations wherein the actual friction coefficient is larger than the predetermined value, the wheel r.p.m. is rather rapidly decelerated to reach a predetermined r.p.m. after a relatively short time from braking operation. At the predetermined wheel r.p.m., the target wheel r.p.m. is determined and the brake control system becomes operative. By entering into the skid controlled state a relatively short period after application of the brake, the target wheel r.p.m. is determined based on a relatively high vehicle speed. Therefore, the braking distance is longer than that required. To the contrary, if the actual friction coefficient is smaller than the predetermined value, it takes a relatively long period to decelerate the wheel r.p.m. to the predetermined target speed value. Thus, if a target speed is determined which is considerably lower than the vehicle speed, it is possible to cause locking of the wheel.
For effectively and satisfactorily controlling vehicle skid due to the vehicle brake system, it is required to determine the most suitable deceleration rate corresponding to friction between the wheel tread and the road surface. As stated above, the friction between the wheel tread and the road surface is maximized for a wheel decelerating rate approximately 15% lower than the vehicle speed. Therefore, by determining the peak coefficient of friction in each cycle of skid control operation and by controlling the ratio of applying and releasing the brake fluid pressure to the wheel cylinder corresponding to detected peaks of the coefficient of friction, the vehicle braking operation can be effected most effectively and satisfactorily.
Various approaches may be used in order to determine the friction condition between the wheel tread and the road surface and for discriminating the peak of the friction coefficient. For example, the friction coefficient may be determined as a function of wheel acceleration, wheel load and brake torque, or by determining the deceleration ratio of the wheel r.p.m. based on measured wheel r.p.m. In a method for discriminating the peak of the friction coefficient, the wheel r.p.m. is measured when the braking pressure is relieved in order to recover the wheel r.p.m. Based on the measured wheel r.p.m., the acceleration ratio of the wheel r.p.m. is determined during the wheel r.p.m. recovery period in which brake pressure is released. Since, in this manner, the wheel r.p.m. is not seriously affected as the result of a determination of the acceleration ratio, the friction between the wheel tread and the road surface can be more accurately determined.